REGIME/REGIMEN/REGIMENT

Some people insist that “regime” should be used only in reference to
governments, and that people who say they are following a dietary regime
should instead use “regimen”; but “regime” has been a synonym of
“regimen” for over a century, and is widely accepted in that sense.

However “regiment” is an error in this sense. The only way you could follow a strict regiment would be to march behind a highly disciplined military unit. Your diet or exercise routine is not a “regiment.”